It would help if you told us which radius server you're using...
For radiusd-cistron (which we use at the ISP I work for), the
configuration files are in /etc/raddb, documentation can be found in
/usr/share/doc/radiusd-cistron.

Same places for radiusd-freeradius package.
Read /usr/share/doc/radiusd-*/* for any related reason.
Read/write docs important forever
SH